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									                                              TRANSFORMER INRUSH PHENOMENON

                             1. INTRODUCTION

It is recommended that differential protection be used for the protection of transformers
of 10MVA (self-cooled) and higher. Differential protection generally is considered the
best protection for transformers. However, inrush current due to transformer energization
exists mostly only in one winding of the transformer; therefore, the relay sees the
energization condition as a fault. To improve security while maintaining the required
levels of dependability, many restraint methods have been proposed to block the
operation of the differential element due to the inrush current. It is very well known that a
transformer will experience magnetizing inrush current during energization. Inrush
current occurs in a transformer whenever the residual flux does not match the
instantaneous value of steady-state flux.

PEPS EEE DEPARTMENT AJCE                                                                   1
                                              TRANSFORMER INRUSH PHENOMENON


Transformer inrush is described as the phenomenon:
"When a transformer is suddenly energized with full system voltage , a random saturation
phenomenon may occur, which is usually referred to as an inrush current" The inrush
current has a high degree of asymmetry and is harmonically rich due to it being created
by saturation of the transformer's magnetic circuit. In addition, there is a large DC offset
component which contributes significantly to the peak component. The inrush current
typically lasts for tens of cycles and as such has a prolonged effect on the voltage of the
network it is being energized from. The degree to which it depresses the network voltage
is dependent on the short-circuit strength of the network relative to the rating and the
inrush characteristic of the transformer in question

                                                                Time (sec)

PEPS EEE DEPARTMENT AJCE                                                                  2
                                              TRANSFORMER INRUSH PHENOMENON

                3. CAUSES OF INRUSH CURRENT

Inrush current in transformers results from any sudden change of the magnetizing
voltage. Although usually considered a result of energizing a transformer, the
magnetizing inrush may be also caused by
1. External faults,
2. Voltage recovery after clearing an external fault,
3. Out-of-phase synchronizing of a connected generator.
Since the magnetizing branch representing the core appears as a shunt element in the
transformer equivalent circuit, the magnetizing current upsets the balance between the
currents at the transformer terminals, and is therefore experienced by the differential
relay as a "false" differential current. The relay, however, must remain stable during
inrush conditions. In addition, from the standpoint of the transformer life-time, tripping-
out during inrush conditions is a very undesirable situation (breaking a current of a pure
inductive nature generates high over voltage that may jeopardize the insulation of a
transformer and be an indirect cause of an internal fault).

PEPS EEE DEPARTMENT AJCE                                                                 3
                                                TRANSFORMER INRUSH PHENOMENON


Size of a transformer
The peak values of magnetizing inrush currents are higher for a smaller transformers
while the duration of this current is longer for larger transformers. The time constant for
the decaying current is in the range of 0.1 of a second for small transformers(100kVA
and below) and in the range of 1 second for larger units.

Impedance of the system from which a transformer is energized
The inrush current is higher when the transformer is energized from a powerful system.
Moreover, the total resistance seen from the equivalent source to the magnetizing branch
contributes to the damping of the current. Therefore, transformers located closer to the
generating plants display inrush currents lasting much longer than transformers installed.

Magnetic properties of the core material
The magnetizing inrush is more severe when the saturation flux density of the core is
low. Designers usually work with flux densities of 1.5 to 1.75 Tesla. Transformers
operating closer to the latter value display lower inrush current.

Remanence in the core
Under the most unfavorable combination of the voltage phase and the sign of the
remanent flux, higher remanent flux results in higher inrush currents. The residual flux
densities may be as high as 1.3 to 1.7 Tesla.

Moment when a transformer is switched in
The highest values of magnetizing inrush current occurs when the transformer is switched
at the zero transition of the winding voltage, and when in addition, the new forced flux
assumes the same direction as the flux left in the core. In general, however , the
magnitude of the inrush current is a random factor and depends on the point of the

PEPS EEE DEPARTMENT AJCE                                                                 4
                                              TRANSFORMER INRUSH PHENOMENON

voltage waveform at which the switchgear closes, as well as on the sign and value of the
residual flux. It is approximated that every fifth or sixth energizing of a power
transformer results in considerably high values of the inrush current.

Way a transformer is switched in
The maximum inrush current is influenced by the cross-sectional area between the core
and the winding which is energized. Higher values of inrush current are observed whwn
the inner (having smaller diameter) winding is energized first. It is approximated that for
transformers with oriented core steel, the inrush current may reach 5-10 times the rated
value when the outer winding is switched –in first, and 10-20 times the rated value when
the inner winding is energized first. Due to the insulation considerations, the lower
voltage winding is usually wound closer to the core, and therefore, energizing of the
lower voltage winding generates higher inrush currents. Some transformers may be
equipped with special switchgear which allows switching-in via a certain resistance. The
resistance reduces the magnitude of inrush currents and substantially increases their
damping. In such a case, the operating requirements for the differential protection are
much more relaxed. In contrast, when a transformer is equipped with an air-type switch,
then arcing of the switch may result in successive half cycles of magnetizing voltage of
the same polarity.
The consecutive same polarity peaks cumulate the residual flux and reflect in a more and
more severe inrush current. This creates extreme conditions for transformer protection.

PEPS EEE DEPARTMENT AJCE                                                                  5
                                                TRANSFORMER INRUSH PHENOMENON

                   5.REDUCTION TECHNIQUES

Probably every utility has experienced a false operation of a differential relay when
energizing a transformer bank. Over the years, many different methods of preventing
differential relay operation on inrush have been implemented.
1. Ignore it. That is, if it can be verified that there was actually no fault (via visual
inspection, no sudden pressure relay operation, no oscillograph operations in the area,
etc.) in the transformer, the bank is simply reenergized. Should the bank trip again, in
some cases it has been reported that the differential trip cutout switch would be opened,
the bank energized, the differential relay contacts verified to be open, and the cutout
switch closed.
2. Desensitize the relay by:

present on energization. On inrush, it is assumed that the bus voltage will not drop
appreciably; the auxiliary relay will be picked up, restraining the differential element
from operating. If a fault exists, the voltage will drop, the auxiliary relay drops out,
allowing the differential element to operate.
3. Use slow-speed induction-type relays with long time and high current settings.
4. Power differential method - This method is based on the idea that the average power
drawn by a power transformer is almost zero on inrush, while during a fault the average
power is significantly higher.
5. Rectifier relay - This method takes advantage of the fact that magnetizing inrush
current is in effect a half-frequency wave. Relays based on this method use rectifiers and
have one element functioning on positive current and one on negative current. Both
elements must operate in order to produce a trip. On inrush, the expectation is that one
element only will operate, while on an internal fault, the waveform will be sinusoidal and
both elements will operate.

PEPS EEE DEPARTMENT AJCE                                                                6
                                                TRANSFORMER INRUSH PHENOMENON

6. A variation of method 5 is the method of measuring “dwell-time” of the current
waveform, that is, how long it stays close to zero, indicating a full dc-offset, which it uses
to declare an inrush condition. Such relays typically expect the dwell time to be at least ¼
of a cycle, and will restrain tripping if this is measured.
7. Another unique method uses the flux-current relationship of the transformer
to provide restraint.
8. Harmonic current restraint - This is the most common method and is discussed in more
detail below.
An important feature of this inrush current is that it is evident that the currents are not
pure fundamental frequency waveforms. Past research has shown that magnetizing inrush
produces currents with a high second harmonic content, with relatively low third
harmonic content. Relay designers have taken advantage of this fact, along with the fact
that internal fault currents have relatively low harmonic content. Relays have been
designed with fixed second harmonic restraint thresholds that will restrain tripping if the
input currents have a certain level of harmonic current, and allow tripping if the second
harmonic content is below that particular threshold. Different manufacturers chose
different thresholds.

PEPS EEE DEPARTMENT AJCE                                                                    7
                                                TRANSFORMER INRUSH PHENOMENON


For EHV transformers, the relay current and time ratings necessary to ensure stability on
the magnetizing inrush current caused by switching-in the transformer are not adequate
for providing high speed protection.
       A high speed biased differential relay incorporating a harmonic restraint feature is
immune to the magnetizing inrush current. The magnetizing inrush currents have high
component of even and odd harmonics (about 63% of 2ND harmonics and 26.8% of 3RD
harmonics) while harmonic component of short-circuit current is negligible. The use of
these facts is made for restraining the relay from operation during initial current inrush.
The harmonic restraint differential relay is sensitive to fault currents but is immune to the
magnetizing currents. The operating coil of the relay carries only fundamental component
of current only while the restraining coil carries the sum of the fundamental and harmonic
       The restraining coil is energized by a direct-current proportional to bias winding
current as well as the direct current due to harmonics. Harmonic restraint is from the
tuned circuit that allows only the fundamental component of current to enter the
operating circuit. The dc and higher harmonics (mostly second harmonics) are diverted
into the rectifier bridge feeding the restraining coil. The relay is adjusted so that it will
not operate when the harmonic current exceeds 15% of the fundamental current. Both the
dc and higher harmonics are of large magnitude during magnetizing inrush.
   The relay may fail to operate due to harmonic restraint feature if an internal fault has
considerable harmonics, that may be present in the fault current itself due to an arc, or
due to saturation of CT. Also, if a fault exists at the instant of energization of transformer
harmonics present in the magnetizing current may prevent the operation of the relay. This
problem can be overcome by providing instantaneous overcurrent relay in the differential
circuit which is set above the maximum inrush current but will operate in less than one
cycle on internal faults. Thus fast tripping is ensured for all internal faults.

PEPS EEE DEPARTMENT AJCE                                                                    8
                                             TRANSFORMER INRUSH PHENOMENON


The majority of relay manufacturers use the terms of harmonic restraint, harmonic
inhibit, or harmonic blocking interchangeably. Although there are some variations in
implementation, the differential relay will not operate when

                                                                            Equation 1
          - Second harmonic current
          - operating current

In order to overcome the challenge of secure differential protection with low harmonic
component in inrush current in new transformers, various harmonic restraint methods
have been studied.

7.1 Per Phase Method
This is the earliest and simplest harmonic restraint method. Equation applies to phase A,
B and C separately. The restraint algorithm in each phase is independent and parallel.
Since each phase has different residual flux and is energized at a different angle, each
phase will most likely have different harmonic levels. When the second harmonic ratio
for a particular phase is above a preset level, the percent differential operation on that
phase will be inhibited. Both experience and analysis show that it is possible to have low
second harmonic ratio for one phase during transformer energization. Differential
operation for a phase with small second harmonic ratio may cause an undesirable trip for
a three-phase transformer. The differential protection with per-phase harmonic restraint
method is most dependent but least secure.

PEPS EEE DEPARTMENT AJCE                                                                 9
                                              TRANSFORMER INRUSH PHENOMENON

7.2 Cross-blocking Method

The harmonic detection for cross-blocking method is the same as that of per phase
method. The only difference is that the restraint signal from one phase will inhibit
differential operation for all other phases. This improves security by allowing the phase
with the low harmonic ratio to be cross-blocked by a phase with a higher ratio preventing
possible false trips. However, inrush current is generally several times the rated current,
and the insulation bears the most severe mechanical stress during energization. If there is
an internal fault in one phase or two phases, the high second harmonic ratio in a healthy
phase may block the percent differential protection until the fault spreads to all three
phases. The differential protection with cross blocking harmonic method is very secure
but least dependable. Two-out-of-three restraint method is a slight variation of the cross-
blocking one. The blocking of differential operation will need at least two phases to
detect sufficient harmonic level. The disadvantage of this variation basically stays the
same as that of cross-blocking method.

7.3 Percent Average Blocking Method

The harmonic ratio for a percent average blocking method is the average of the second
harmonic ratio of the three phases

Compared to the cross-blocking method and two-out-of-three method, this method
improves the security of differential protection to a certain degree. The differential
operation may be restrained when there is a true single phase fault during energization,
provided that there are large harmonic ratios in the remaining healthy phases. This would
cause a concern on the dependability in the differential protection.

PEPS EEE DEPARTMENT AJCE                                                                10
                                                     TRANSFORMER INRUSH PHENOMENON

7.4 Summing-type Harmonic Sharing Method

A summing-type harmonic sharing restraint method greatly improves the dependability of
the differential protection during an internal fault. The shared second harmonic
component is defined as

                I 2nd sum = | I op2nd A| + |I op2nd B| + |I op2nd C|      Equation 2

 The second harmonic ratios are then calculated per phase based on the shared sum and
the fundamental component of the operating current in each phase is,

                                                                           Equation 3

                                                                           Equation 4

                                                                           Equation 5

2nd Ratio A, B, C – Second harmonic ratio of phase A, B, C
I 2nd sum        - Sum of second harmonic current of three phases
I op fund A, B, C – Operating current fundamental component of phase A, B, C

PEPS EEE DEPARTMENT AJCE                                                           11
                                              TRANSFORMER INRUSH PHENOMENON

The harmonic restraint with summing-type harmonic sharing is illustrated in the figure


In this method, the magnitudes of the second harmonic from three-phases are summed
together to create a single harmonic signal, which is shared in the calculation of the
second harmonic ratio in each phase. If there is an internal fault in one phase during
energization, the large fundamental operating current would result in low harmonic ratio
and, thus, no inhibit is generated from the faulted phase. Therefore, a three-phase
transformer would be tripped during an internal fault. If one phase experiences a very low
second harmonic ratio, the shared harmonic calculated from equation will be large and
the second harmonic ratios calculated from equation will be large enough to restrain the
differential protection from a false operation.

PEPS EEE DEPARTMENT AJCE                                                                 12
                                             TRANSFORMER INRUSH PHENOMENON


                      A Low Second Harmonic Inrush Event

Fig. illustrates a delta/star transformer connected to a radial distribution system. While
this transformer is energized with load open, there is a station service transformer
connected to the transformer secondary but outside the differential zone protection. When
the main transformer is energized, the station service transformer also will be energized.
This installation had problems with tripping during energization, and the user switched to
a numeric relay. High voltage side CT is Y-connected and low voltage side CT is delta-
connected for external angle compensation. A set of COMTRADE data files was
downloaded from an energization event, which is illustrated in the figures below.

                      FIGURE 8.1 -   A DELTA/STAR TRANSFORMER [1]

PEPS EEE DEPARTMENT AJCE                                                               13
                                             TRANSFORMER INRUSH PHENOMENON

The per-phase second harmonic ratios for all three phases are illustrated in Figure
There is very low second harmonic ratio in phase B. For differential protection using
per-phase harmonic restraint, security likely is a problem. The low second harmonic
ratio in phase B was probably the reason this transformer had problems with tripping
during energization.


PEPS EEE DEPARTMENT AJCE                                                                14
                                            TRANSFORMER INRUSH PHENOMENON

When considering second harmonic cross-blocking for this application, the second
harmonic ratios in phase A and C are large, as seen from Figure, differential tripping of
phase B will be cross blocked.
Figures indicate the result of applying average second harmonic ratio and summing-type
harmonic sharing restraint methods. In both cases we observe the differential protection
would be secure for this low harmonic energization.

                  FIGURE 8.3 - AVERAGE 2ND HARMONIC RATIOS [1]

PEPS EEE DEPARTMENT AJCE                                                              15
                                 TRANSFORMER INRUSH PHENOMENON

                    SUMMING- TYPE HARMONIC SHARING [1]

PEPS EEE DEPARTMENT AJCE                                    16
                                             TRANSFORMER INRUSH PHENOMENON


Security is of concern in differential protection during transformer energization. Inrush
current occurs mostly only in one side of the transformer and could cause a false
differential trip. A common method is to use the second harmonic information in inrush
current to secure differential protection when energizing transformers. An analysis of
factors like residual flux, saturation flux, and energizing angle on the second harmonic
ratio in inrush current is provided in this paper. Higher residual flux and/or lower
saturation flux in transformers may result in high differential operating current and lower
second harmonic ratio, which is likely to cause a security concern in differential
protection during transformer energization. It is well known that modern transformers
may experience very low second harmonic ratios. Per-phase harmonic restraint provides
best dependability but worst security. Harmonic cross-blocking provides best security but
worst dependability. Average-percent harmonic restraint may cause an unexpected
blocking when there is a true fault during energization. The restraint with summing-type
harmonic sharing provides very good dependability while maintaining the security for
differential protection.

PEPS EEE DEPARTMENT AJCE                                                                 17

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